Servo system



F. F. OFFNER Sept. 4, 1956 SERVO SYSTEM Filed Jan. 24, 1951 m O7 0%@ #59.4. BYMJW@ anu.-

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ATTORNEYS sERvo SYSTEM p Franklin F. olrner, chicago, nl.

' Application January 24, 1951, serial No. 207,621

4 claims. (C1. 251-25) This invention relates to hydraulic servo systems and more particularly to a system including a throttle valve inserted in a pipe line for varying the rate of liquid flow through the line and a pilot valve for controlling the setting of the throttle valve.

One object of the invention is to provide a servo system of the hydraulic type consisting of a throttle valve controlled by a pilot valve featuring a new principle` of I improved form of throttle valve controlled by the pilot t valve, the two valves working in conjunction withv each other to provide a servo system of exceptional accuracy. The foregoing as well as other objects and advantages inherent in the invention will become more apparent from the folowing detailed description of a preferred embodiment for the invention and the related drawings in which:

Fig. 1 is a view of the servo system with'the throttle and pilot valves shown in central longitudinal sections; Fig. 2 is a view in elevation of the rotatable sleeve employed in the pilot valve assembly; and

Figs; 3-5 are transverse sections taken on lines 44 and'S-S respectively of Fig. 1.

With reference now to the drawings, the servo system is comprised generally of a throttle valve T, an inlet pipe 8 to the valve, an outlet pipe 9 from the valve, a pump 10 of the constant delivery type in the pipe 8 to force liquid uid such as fuel through the valve into outlet pipe 9 under pressure and a pilot valve P for controlling the setting of the throttle valve T.

The throttle valve T consists of a cylinder 11 containing two identical sets of diametrally disposed ports 12, 12 and 13, 13 and a piston 14 slidable within the cylinder across the entrance to such ports to vary the port opening. The ports 12, 12 are manifolded into the inlet pipe 8 and hence the liquid fuel under pressure enters the valve cylinder 11 below piston 14 through these ports. The dual opposed port` arrangement is `not absolutely necessary but is preferred so as to balance out the lateral forces on the piston attributable to pressure of the incoming liquid.

The other set of opposed ports 13, 13 are displaced 90 from the ports 12, 12 but are located in the same position as the latter in relation to the cylinder 11. That is, the centers of all four ports are located in a common plane perpendicular to the longitudinal axisof the cylinder. Ports 13, 13 are manifolded into the outlet pipe 9. v

Flow of liquid through throttle valve T iscontrolled by varying the position of piston 14 therebyltoY increase 2,761,645 Patented Sept. 4, 1956 ICC or decrease the area of the ports 12, 12 and 13, 13 uncovered by the piston. The position of piston 14 is controlled by admission of the liquid from the pilot valve P into that portion 11a of cylinder 11 above the piston, i. e. at the side of the piston opposite to that which controls the -inlet and outlet ports. The cylinder portion 11a constitutes a servo chamber.

The pilot valve in its preferred form of construction is comprised of an outer cylindrical casing 15 having an end cap 15a secured by screws 15b, an inlet port 16, an outlet port 1,47 and a servo port 18 located intermediate the inlet and outlet ports 16, 17, all the ports-being spaced longitudinally of the casing. Journalled for rotation within casing 15 is a valve sleeve 19 provided with an annular groove 20 aligned with inlet port 16, and another annular groove 21 aligned with outlet port 17. An aperture 22 leads from the base of groove 20 through the wall of sleeve 19 into the bore of the sleeve, an aperture 23 similarly leads from the base of groove 21 into the bore "of sleeve 19, and a port 24 also leading into the bore of sleeve 19 through the sleeve wall is disposed in alignment with the servo port 18 in casing 15. A pipe connection 26 leads from inlet pipe 8 to port 16, a second pipe connection 27 leads from outlet pipe 9 t-o port 17, and a third pipe connection 28 leads from servo port 18 to a port 29 leading to the servo chamber 11a in the throttle valve cylinder.

Since the liquid ows from pipe 8 to pipe 9 through throttle valve T, it is at a higher pressure in pipe 8 than in pipe 9. The liquid in inlet pipe 8 is thus used asa source of high pressure for servo control and is led to the interior of pilot valve sleeve 19 through pipe 26, groove 20 and aperture 22. In a similar manner, the liquid in outlet pipe 9 is used as the source of low pressure for the servo system, and liquid is returned to it from the interior of sleeve 19 through aperture 23,

r v groove 21 and pipe 27.

Slidable axially in the bore of rotatable sleeve 19 is the valve stem 32 having end lands 33, 34 and an intermediate land 35. Thc function of land 35 is to selectively control ow of the liquid uid between servo chamber 11a and the pipe connections 26, 27. If valve stem 32 is shifted to the right from the illustrated neutral position, liquid in the pipe 26 at the high pressure of the system, and which is constantly in communication with the interior of the rotatable pilot valve sleeve 19 between the lands 33, 35 of the valve stem because of the ,annular groove and aperture arrangement described, Will be allowed to flow through pipe connection 28 into servo chamber 11a each time that the rotating-sleeve 19 brings the port 24 into registry with the servo port 18.

In a similar manner, if the valve stem 32 is shifted to the left from the illustrated position, pipe connection 27 is placed in communication with pipe connection 28 for a brief period during each revolution of sleeve 19,

^ and liquid leaves servo chamber 11a, passing into outlet pipe 9 via the communication channels thus established.

It will be evident that the percentage of the total time that the liquid can ow from servo chamber 11a through pipe 28 into the interior of the pilot valve sleeve 19, or

' vice versa, will depend upon the size of port 24 at the outer surface of sleeve 19 and of port 18. These are preferably made quite small so that fluid can flow for only a small fraction of the elapsed time required for one revolution of sleeve 19. While port 24 may be made of uniform size throughout its length, as illustrated, the size of port 24 at the inner surface of sleeve 19 is made larger than that at the outer surface of the sleeve to permit a comparatively large amount of instantaneous uid ow when the pilot valve is displaced. During the remainder of the time, pipe 28 is eectively sealed olf.

3 For. example, it pipo. Zit communicates. with the, interior of sleeve 19 10% of the time, and is sealed ol 90% 0f the time, during the latter fraction no liquid can leave servo chamber 11a. Thus the servo piston; 14' in, the throttle valve T will be eectively subject t hydraulic lock, and will be substantially xed, even though,` there be leakages around the throttle piston such that the piston would normallyA tend to drift in position. During the remaining of the time, the motionA of the; pilot valve stem 32, andthe size ofv porting, ismade suicient that an adequate travel rate of throttle piston 14 may be obtained. Thus by such design of the pilot valve, the

accuracy of the servo system is increased, under the l assumed 9 0-10 ratio of porting, by a factor; offy 1 0. Rotation of thasloove1f9 inthe pilot valvev has the addif tionaladvantageof preventing development of frictional forces in the pilot valve, and isipreferable to chopping the liquid flow by rotation, of the valve stern relative to the, sleeve because. it Provides a. more complete shutott of the liquid during the periods when the chopperl is supposed to be closed.

In the introduction one stated object of the invention isto provide aservo system havinga high degree' of stability without the necessity for maintaing close tolerances between the various component parts. Onefactor in achieving thisisithe construction ofthe throttle valve T, The opposed inlet ports 12, 12 are identical in size and shape, and moreover are of the same` sizel and` shape as the two opposed outlet p orts 13, 13. Consequently when liquid ows through the valve T, one-half-` the pressure drop through the valve will occur at the inlet ports 12, 12 and the other, half of the pressure drop occurs at the outlet ports 13, 13. tion 11b of cylinder 11 below piston 14-,will be just mid.- Way between the pressures in the inlet and outlet pipes 8, 9; This condition will obtain, independent of the position of piston 14 in its cylinder, provided only that some liquid continues to flow through the valve which can be assured if desired by insertion of a stop inthe valve for the piston so as to prevent it from completelyobstructing thek inlet and outlet ports.

In order that the throttle valve-piston 14 shall remain stationary at; any selected setting, it is necessary that the liquid pressure in servo chamberl 11a be equal to the pressure below the piston inthrottle chamber 11b since the piston 1,4 in the illustrated embodiment is of univ` forrndiameter. This will beY true if thestem 32v of pilot valve P is in such a position that pipe 28 communicates equally with thechambers in the pilot valve connected withpipes 26 and, 27. That is, land 35 must be-in'.such relation to servo port- 24 that equal leakages'. flow-between pipe 2,8 and pipe26, as flow-between pipe 28 and pipe 27. Thatis, thev pressure in pipe 28 and: thus. in chamber 11a, will be midway betweenl the pressures in tubes 26 and 27, i. e. midway between the pressures in pipes8 and 9. As illustrated, land 3S is of the same width as port 24 and in general, leakage balance will occur.l whenthe-.landl is centered on the port which is the position depicted. However land 35 can be narrower or wider than port 24 without altering the principle of the invention since there will always besome leakage.

It is my theory that the leakage between pipes 2,6 and 28 will be equal to that between pipes 28 and 27 when the pressure drops occurring in the pilot valve between these pairs of pipes arein the sameA ratio as the ratio of the pressuredrops occurring inthe inlet orifices 12, 12, and the outlet orices V13, 13 of the throttle valve.

Since the pressure in chamber 11a is the same as that in chamber 11b,r there will be no liquidow'between themeven though there be clearance between piston 14 and the wallV of cylinder 11. Thusl the clearance be tween this piston and its cylinder is notl critical. Likewise, such equalityfof'pressure is maintained by` a con-y stantposition. of; the-pilot valve stemZ, independentot Thus the pressure in that porthe position. of throttle. piston. 1.4 so. long. as. there. are. equal pressure drops in ports 12, 12 and 13, 13.

Equality of pressure drop, in these ports is most simply obtained but it should be mentioned that it is not necessary that equal pressure drops in ports 12, 12' and 13, 13 occur. They can be in any desired fixed ratio. However, if the selected, pressure drops as between the inlet and outlet ports of the throttle valve be other than equal, a corresponding, change must be made in the equilibrium position of theA pilot valve to maintain equality of leakage between pipes 26'and- 28 as between piposl?.` and. 2.8.,dospitotho. existence of, a.d.if ference between the two pressure drops.

Because of thepressure differentialI between the liquid in servo pipes 26, 27 and pipe 28 there will always be a tendency for liquid to leak past the land 35 in pilot valve P either to or from servo chamber 11a. With the type ofI servo; systemnhereinA described whereinl the pressursis. porto-. 1 into the pilot valve, this-.tander1c.yl islarsoly balanced out. With less, stable systems, however, such leakagsloau be. a source.y of; material,` error.

Operation of the system should; be clear from4 the above. dasoriptiont However by Way ofzabrief summary, movement-V Qfthepilotvalve stem 32 to the right-from the` illustrated neutralv position` will effect at 'ow of. liquid intolseryo chamber 11a since the pressure inA the lattepwill thion-vv be; greater thanv that in chamber. 11b.. Piston 1,45 isthus caused to move further downward in the-cylinder;11 y and close off more offthe port openings1- 2, 1 2 and- 13, 13K thus effectingga reduction in rate Qfgliqllid' low throughvalve "Il, Conversely/movement of valve 32 to the left from the illustrated position will effecta flow of liquidv from.servo,charnber 11a since-then the pressure in. thelatter will beless than; that inchamber 11b. Pistonl 14; is thus ca used4 to move toa more upward position and uncover more of the port1openings 1.2i,4 112i and 13, 1 3' thus effecting an increasein rateot'. liquid,4r ow through valve T. Piston 14 will continue to moueintheselecteddirection solong as thestem 3.2:of: thelpilot valve; is heldin a displaced; position and is stopped'byv returning thepilot valve.` stem. 32; tothe neutral position.

In conclusion, I wish it. to beunderstood that while the illustrated construction constitutes a.- preferred embodiment, for. my, invention,l various minor changes. may be made in the construction and arrangementfof thev compiment.v parts: inl thesystem. withoutv however.l departing from the spirit and-'scope oftheninvention as dened-.in the appended-i claims.

Ir claimz.

1, In. a1 hydraulic.A servo system, the combination conif praising; a. throttle-valve adaptedtobel inserted inf a: pipe linetocontroldiquid fluid-.flowing through the line under pressure, said valve: .including acasing providing ai throttle chamber having alhighpressure tluidfinlet: port thereto and; a, lowen pressurev fluidy outlet: pox-tf therefrom, a pistonslidableinsaidcasngra'cross` said ports to'. adjust: the degree of port'- opening,A the space in. said` casing at thc pistonA end` opposite that controlling' said.. ports consti.-

tuting a ser'voA chamber; a pilotL valve having a highv pressure inletfport, a llower pressureoutlet: port, aservo andiv a valvemember forestablishing; communica.- tion selectively-f between. said inlet port; and servo l port or between said-.outlet port and servoportga pipe connectiongbetweenfthe servoi porton said*l pilot' valvefantl the servo chamber in said throttle valve-L andzpipe-eon: motions. leading respectively-from the inlet and outlot ports. on sf'tidspilotA valveV to theypipolinev at the.v high. andlow. pressurosides; o..f;said;th.rottle.va;1ve, .saitlinlotrand outlet ports otsaid. throttle. valve .being so. contour-cdas to. provide pressure drops at'. said ports remaining. in' conf stante-.proportion 1 to. each. other; as said. throttle valve. nis.- ton is displaced whereby the pressure difference be:l tween.. said throttle: Chamber; and. the inlet.. Side to.-I said throttle-.valve romains; intxed ratio. v te. the.- .Prossurez diff ference between said throttle chamber and the outlet side of said throttle valve.

2. In a hydraulic servo system for use in controlling flow of liquid under pressure in a pipe line, the combination comprising; a throttle valve including a servo chamber, a throttle chamber, inlet and outlet ports for the liquid to and from said throttle chamber and a piston disposed in said throttle and servo chambers and slidable to cover and uncover said ports; a pilot valve including a casing having inlet, outlet and servo ports and a piston slidable in said casing across said ports for controlling liquid ow between said ports; iluid connections leading from the pipe line at the inlet and out let sides of said throttle valve to the inlet and outlet ports respectively of said pilot valve; and a iluid connection leading from the servo port of said pilot valve to the servo chamber of said throttle valve, said inlet and outlet ports of said throttle valve being so contoured as to provide pressure drops at said ports remaining in constant proportion to each other as said throttle valve pis= ton is displaced, whereby the pressure difference between said throttle chamber and the inlet side to said throttle valve remains in fixed ratio to the pressure difference between said. throttle chamber and the outlet side of said throttle valve.

3. A hydraulic servo system as defined in claim 2 wherein said inlet and outlet ports of said throttle valve are alike thereby to establish in said throttle chamber a iluid pressure midway between the iluid pressures at the said inlet and outlet sides of said throttle valve.

4. A hydraulic servo system as defined in claim 2 wherein said inlet and outlet ports of said throttle valve comprise respectively a pair of oppositely disposed inlets, and a pair of oppositely disposed outlets arranged normal to said pair of inlets.

References Cited in the tile of this patent UNITED STATES PATENTS 175,485 Miracle Mar. 28, 1876 674,606 Forsyth May 21, 1901 770,866 Mullin Sept. 27, 1904 1,196,121 Larsen Aug. 29, 1916 1,771,511 Quelch July 29, 1930 1,904,475 Kissing Apr. 18, 1933 2,012,978 Veenschoten Sept. 3, 1935 2,204,640 Woodward June 18, 1940 2,219,229 Kalin Oct. 22, 1940 2,313,438 Hoelscher Mar. 9, 1943 2,331,817 Turchan Oct. l2, 1943 2,542,765 Gillespie Feb. 20, 1951 FOREIGN PATENTS 679,003 France 1930 

